Method and apparatus for computing dummy feature density for chemical-mechanical polishing

ABSTRACT

One embodiment of the present invention provides a system that computes dummy feature density for a CMP (Chemical-Mechanical Polishing) process. Note that the dummy feature density is used to add dummy features to a layout to reduce the post-CMP topography variation. During operation, the system discretizes a layout of an integrated circuit into a plurality of panels. Next, the system computes a feature density and a slack density for the plurality of panels. The system then computes a dummy feature density for the plurality of panels by, iteratively, (a) calculating an effective feature density for the plurality of panels using the feature density and a function that models the CMP process, (b) calculating a filling amount for a set of panels in the plurality of panels using a target feature density, the effective feature density, and the slack density, and (c) updating the feature density, the slack density, and the dummy feature density for the set of panels using the filling amount. In one embodiment of the present invention, the iterative process is guided by a variance-minimizing heuristic to efficiently select the set of panels and assign/remove dummy density to the set of panels to decrease the effective feature density variation.

RELATED APPLICATION

This application is a divisional application of application Ser. No.10/997,396, Attorney Docket Number SNPS-0565, entitled “METHOD ANDAPPARATUS FOR COMPUTING DUMMY FEATURE DENSITY FOR CHEMICAL MECHANICALPOLISHING,” by inventors Xin Wang, Charles C. Chiang, and Jamil Kawa,filed on 24 Nov. 2004, which claims the benefit of provisionalApplication No. 60/524,786, Attorney Docket Number 0565, entitled“Min-variance iterative method for fast smart dummy feature densityassignment in chemical-mechanical polishing,” by inventors Xin Wang,Charles Chiang, and Jamil Kawa, filed on 24 Nov. 2003. Attorney DocketNo. SNPS-0565 Inventors: Wang et al.

BACKGROUND

1. Field of the Invention

This invention relates to the process of fabricating semiconductorchips. More specifically, the invention relates to a method andapparatus for computing dummy feature density for performing dummyfilling that improves the quality of a Chemical-Mechanical Polishing(CMP) process.

2. Related Art

The miniaturization of integrated circuits has been a key driving forcebehind technological innovations. Miniaturization of transistors andinterconnects has occurred because of the rapid advances in variousfabrication technologies, such as “Chemical Mechanical Polishing” (CMP).CMP is used to reduce topography variation of a wafer, and today, it hasbecome a critical process in the semiconductor fabrication flow. CMPpolishes a wafer surface to create a flat (planarized) surface.Specifically, CMP combines the chemical removal effect of an acidic orbasic fluid solution with the “mechanical” effect provided by polishingwith an abrasive material.

Today, CMP is used extensively in the semiconductor fabrication flow.Although CMP makes the wafer surfaces smoother, it does not completelyeliminate topography variations. If these post-CMP topography variationsare too large, they can cause serious problems during subsequentfabrication steps. For example, these variations can cause defocusingduring photolithography, which can result in low fabrication yields andlarge performance variations.

Moreover, integrated circuits usually contain multiple metalinterconnect layers. Hence, the post-CMP topography variations canaccumulate over several layers and exacerbate these problems.Furthermore, since miniaturization is expected to continue at a rapidpace (as predicted by Moore's Law), reducing post-CMP topographyvariation is expected to become even more crucial in the future. Hence,it is critically important to reduce post-CMP topography variation.

Recent studies have shown that post-CMP topography variation dependsstrongly on feature density. Specifically, it has been shown that thepost-CMP topography variation can be reduced by decreasing thenon-uniformity of feature density. For example, during the aluminuminterconnect fabrication process, the aluminum metal layer is firstdeposited on the wafer and patterned. Next, an oxide layer that servesas the inter-level dielectric is deposited. Since the underlying metallayer topography is usually not smooth after being patterned, the oxidelayer topography is also not smooth. Hence, an oxide CMP process isapplied to smooth the wafer surface topography so that fabricationprocesses that require a flat wafer surface can be performed. The oxidefeature density non-uniformity affects the CMP process quality. It is amajor cause of the post-CMP oxide topography variation.

Non-uniformity of the oxide feature density can be decreased by addingdummy metal features to low metal density regions of a metal layer. Forexample, some foundries use simple rule-base techniques to fill dummyfeatures in low density regions to reduce post-CMP topography variation.Adding dummy features also helps to improve the quality of other CMPprocesses, such as, copper CMP and STI CMP.

Unfortunately, these simple rule-based techniques have many shortcomingsbecause they do not accurately model the complex CMP process. Inparticular, these techniques can sometimes be ineffective because theyadd dummy features at suboptimal locations. Moreover, these techniquescan be inefficient because they add more dummy features than necessary.

Smart dummy filling techniques, on the other hand, use more accurate CMPmodels to determine the amount and location of dummy features.

Smart dummy filling techniques based on LP (Linear Programming) canproduce optimal results in the case of oxide CMP. But, these techniquescan be extremely time-consuming. The computational cost of LP-basedmethods can become prohibitively high for large layouts with multiplelayers. Consequently, LP-based methods are not practical for manyreal-life dummy-filling scenarios.

On the other hand, greedy heuristics and Monte Carlo methods arecomputationally efficient, but generate sub-optimal dummy-fillings.Unfortunately, due to the continuing miniaturization of feature sizes,sub-optimal dummy-fillings are not sufficient to significantly improveyields and reduce performance variations.

Hence, what is needed is an efficient method for computingsubstantially-optimal dummy feature density for a CMP process to reducethe post-CMP topography variation.

SUMMARY

One embodiment of the present invention provides a system that computesdummy feature density for a CMP (Chemical-Mechanical Polishing) process.Note that the dummy feature density is used to add dummy features to alayout to reduce the post-CMP topography variation. During operation,the system discretizes a layout of an integrated circuit into aplurality of panels. Next, the system computes a feature density and aslack density for the plurality of panels. Note that the feature densityfor a panel specifies the density of existing features in the panel andthe slack density for the panel specifies the maximum density of dummyfeatures that can be added to the panel. Furthermore, note that thefeature density and the slack density are computed for each panel. Thesystem then computes a dummy feature density for the plurality of panelsby, iteratively, (a) calculating an effective feature density for theplurality of panels using the feature density and a function that modelsthe CMP process, (b) calculating a filling amount for a set of panels inthe plurality of panels using a target feature density, the effectivefeature density, and the slack density, and (c) updating the featuredensity, the slack density, and the dummy feature density for the set ofpanels using the filling amount. In one embodiment of the presentinvention, the iterative process is guided by a variance-minimizingheuristic to efficiently select the set of panels and assign/removedummy density to/from the set of panels to decrease the effectivefeature density variation. Note that, in general, decreasing theeffective feature density variation decreases the post-CMP topographyvariation. Furthermore, the convergence rate of the dummy featuredensity computation is increased significantly because, in eachiteration, the method updates the dummy feature density for the set ofpanels, instead of updating the dummy feature density for only a singlepanel. Typically, in the first several iterations, the set of panelsincludes almost all of the panels in the plurality of panels. In lateriterations, the number of panels in the set of panels usually decreases.

In a variation on this embodiment, the function that models the CMPprocess is a 2-D filter function.

In a variation on this embodiment, the panel size is less than 1/10 ofthe characteristic length of the 2-D filter function.

In a variation on this embodiment, the system efficiently selects panelsand computes the dummy feature density by adjusting the characteristiclength of the 2-D filter function.

In a variation on this embodiment, the system calculates the fillingamount Δ(i, j) for the set of panels in the plurality of panels byselecting the set of panels using a variance-minimizing heuristic.

In a variation on this embodiment, the system calculates the fillingamount by computing the target effective feature density using theeffective feature density.

In a variation on this embodiment, the system calculates the effectivefeature density by normalizing the function that models the CMP processso that a perfectly smooth feature density results in a perfectly smootheffective feature density that is equal to the feature density.

One embodiment of the present invention provides a system that computesdummy feature density for a CMP (Chemical-Mechanical Polishing) process.Note that the dummy feature density is used to add dummy features to alayout to reduce the post-CMP topography variation. During operation,the system discretizes a layout of an integrated circuit into aplurality of panels. Next, the system computes a feature density and aslack density for the plurality of panels. Note that the feature densityfor a panel specifies the density of existing features in the panel andthe slack density for the panel specifies the maximum density of dummyfeatures that can be added to the panel. Furthermore, in one embodimentof the present invention, the feature density and the slack density arecomputed for each panel. The system then computes the dummy featuredensity for the plurality of panels by, iteratively, (a) adjusting thecharacteristic length of a filter function to obtain an adjusted filterfunction, and (b) updating the dummy feature density for the pluralityof panels using the feature density, slack density, and the adjustedfilter function. Note that the filter function models the CMP process.Furthermore, the computed dummy feature density is used to add dummyfeatures to the layout, thereby reducing the post-CMP topographyvariation. Additionally, adjusting the characteristic length of thefilter function improves the effectiveness of the dummy feature densitycomputation in reducing the post-CMP topography variation.

In a variation on this embodiment, the system updates the dummy featuredensity by (a) calculating an effective feature density for a set ofpanels using the feature density and a function that models the CMPprocess, (b) calculating filling amounts for the set of panels using atarget feature density, the effective feature density, and the slackdensity, and (c) updating the feature density, the slack density, andthe dummy feature density for the set of panels using the fillingamounts. Note that decreasing the effective feature density variationdecreases the post-CMP topography variation. Furthermore, note that, theconvergence rate of the dummy feature density computation is increasedbecause, in each iteration, the method updates the dummy feature densityfor the set of panels, instead of updating the dummy feature density foronly a single panel.

Moreover, one embodiment of the present invention uses an efficientdummy density assignment method that closely integrates with a low-passfilter CMP model. By utilizing the variance-minimizing heuristic, thisembodiment simultaneously assigns dummy densities to multiple panels ineach step of the iteration to quickly achieve a substantially-optimalsolution.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates various steps in the design and fabrication of anintegrated circuit in accordance with an embodiment of the presentinvention.

FIG. 2 illustrates an example of post-CMP topography variation in oxideCMP in accordance with an embodiment of the present invention.

FIG. 3 illustrates how adding dummy features can decrease post-CMPtopography in accordance with an embodiment of the present invention.

FIG. 4 presents a flowchart that illustrates various high-level steps ina smart dummy filling process in accordance with an embodiment of thepresent invention.

FIG. 5 illustrates the process of discretizing an integrated circuitlayout into a set of panels and computing the metal density and slackdensity of each panel in accordance with an embodiment of the presentinvention.

FIG. 6 presents a flowchart that illustrates the process of computingdummy feature density in accordance with an embodiment of the presentinvention.

FIG. 7 presents various effective feature density curves that illustratehow an iterative process can reduce the non-uniformity of the effectivefeature density in accordance with an embodiment of the presentinvention.

FIG. 8 presents a flowchart that illustrates the process of computingdummy feature density by adjusting the filter characteristic length inaccordance with an embodiment of the present invention.

FIG. 9 presents various plots that illustrate the effect of adjustingthe filter characteristic length in accordance with an embodiment of thepresent invention.

DETAILED DESCRIPTION Integrated Circuit Design and Fabrication

FIG. 1 illustrates various steps in the design and fabrication of anintegrated circuit in accordance with an embodiment of the presentinvention. The process starts with a product idea (step 100). Next, theproduct idea is realized using an integrated circuit, which is designedusing Electronic Design Automation (EDA) software (step 110). Once thecircuit design is finalized, it is taped-out (step 140). After tape-out,the process goes through fabrication (step 150), packaging, and assembly(step 160). The process eventually culminates with the production ofchips (step 170).

The EDA software design step 110, in turn, includes a number ofsub-steps, namely, system design (step 112), logic design and functionverification (step 114), synthesis and design for test (step 116),design planning (step 118), netlist verification (step 120), physicalimplementation (step 122), analysis and extraction (step 124), physicalverification (step 126), resolution enhancement (step 128), and maskdata preparation (step 130).

Smart dummy filling can take place during the physical implementationstep 122 or the physical verification step 126. Specifically, smartdummy filling can be implemented in the Astro® product or the Hercules®product from Synopsys, Inc.

Chemical-Mechanical Polishing (CMP)

CMP polishes a wafer surface to create a flat (planarized) surface. Bycreating a flat surface, CMP improves the quality of subsequentprocesses, such as, photolithography. Specifically, CMP combines thechemical removal effect of an acidic or basic fluid solution with the“mechanical” effect provided by polishing with an abrasive material andpolishing pads. Even though CMP reduces topography variation, it usuallycannot completely eliminate topography variation.

CMP can be used in various fabrication processes. For ease ofexposition, the present application describes embodiments of the presentinvention in the context of the oxide CMP process which is applied tothe aluminum interconnect fabrication flow. It will be apparent to oneskilled in the art that the present invention can be easily applied toother types of CMP processes, such as, copper CMP and Shallow TrenchIsolation (STI) CMP.

FIG. 2 illustrates an example of post-CMP topography variation inaccordance with an embodiment of the present invention. (The drawings inFIG. 2 are for illustration purposes only and may not represent anactual chip layout or CMP pad geometry. Furthermore, the post-CMPtopography in FIG. 2 has been exaggerated for the sake of clarity.)

Integrated circuit chips can contain multiple metallization layers thatare separated by oxide inter-level dielectric layers. Oxide CMP isperformed primarily on each of the oxide layers. Specifically, afterforming a layer of metal features 204 and 206 on wafer 202, thefabrication process deposits a layer of oxide 208. Next, the surface ispolished using a CMP polishing pad 210 with polishing slurry (not shown)that is delivered to the surface of the wafer. The next layer of metalwill be deposited on top of the oxide layer. The CMP process planarizesthe oxide surface so that the next metal layer surface is flat after thedeposition. The next metal layer is then patterned with thephotolithography process to form metal interconnect wires. Note that aflat surface topography is required by the photolithography process toavoid significant defocusing.

Note that although CMP reduces topography variation 212, it does notcompletely eliminate topography variation. Furthermore, note that thetopography variation 212 depends on the underlying metal featuredensity. Recent studies have shown that the topography variation can bereduced by decreasing the non-uniformity of the underlying metal featuredensity.

Smart Dummy Filling

Some foundries are using simple rule-base techniques to fill dummyfeatures in low-density regions to reduce post-CMP topography variation.Unfortunately, these simple rule-based techniques have many shortcomingsbecause they do not accurately model the complex CMP process. Thesetechniques can sometimes be ineffective because they add dummy featuresat suboptimal locations. Moreover, these techniques can also beinefficient because they add more dummy features than necessary.

FIG. 3 illustrates how adding dummy features can decrease post-CMPtopography in accordance with an embodiment of the present invention.(The drawings in FIG. 3 are for illustration purposes only and may notrepresent an actual chip layout or CMP pad geometry. Furthermore, thepost-CMP topography in FIG. 3 has been exaggerated for the sake ofclarity.)

Smart dummy filling techniques use more accurate CMP models to add metaldummy features 314 at appropriate locations on the wafer 302. Note thatthese dummy features are in addition to the existing metal features 304and 306. The fabrication process then deposits a layer of oxide 308 andpolishes the surface using a CMP polishing pad 310 and a polishingslurry (not shown). Note that the post-CMP topography variation 312 withdummy features 314 is less than the post-CMP topography variation 212without dummy features.

Note that the illustrations in FIG. 2 and FIG. 3 are based on an oxideCMP process. But it will be apparent to one skilled in the art thatdummy features can be used to decrease post-CMP topography variation inother types of CMP processes, such as, copper CMP and Shallow TrenchIsolation (STI) CMP.

FIG. 4 presents a flowchart that illustrates various high-level steps inthe smart dummy filling process in accordance with an embodiment of thepresent invention.

The process starts by performing layout density acquisition (step 402),which involves discretizing the integrated circuit into a set of panelsand computing the metal density and slack density for each panel.

FIG. 5 illustrates the process of discretizing the integrated circuitinto a set of panels and computing the metal density and slack densityof each panel in accordance with an embodiment of the present invention.Specifically, the chip layout 502 is discretized into a set of panels506. (For ease of exposition, the process is illustrated for a singlemetal layer.) A panel, such as panel 506, can contain existing features512, such as interconnect wires 508 and 510. Panel 506 can also containa slack region 514 which does not contain any existing features andwhich can be used for adding dummy features, such as dummy features 516.The system computes the density of existing features 512 in panel 506.The system also computes the slack density for panel 506, whichindicates the maximum density of dummy features 516 that can be added tothe slack region 514 in panel 506.

Next, the system assigns dummy feature density (step 404). In this step,the system uses a CMP process model to compute the dummy feature densityto be added to the slack region 514 in panel 506. Note that adding dummyfeatures can reduce the post-CMP topography variation. The system thenperforms dummy feature placement (step 406) based on the computed dummyfeature density and design rules. For example, the system may placedummy features 516 based on the computed dummy feature density. Once thenew layout with dummy features is sent to the foundry for fabrication,the dummy features are produced and CMP is used to planarize the waferthat contains dummy features.

The most challenging step in this smart dummy-filling process is dummyfeature density assignment 404 (which includes modeling the CMPprocess). Hence, utilizing an efficient dummy feature density assignmentalgorithm with the right CMP model is critical for successfully solvingthe smart dummy-filling problem.

One embodiment of the present invention uses a variance-minimizingiterative density assignment method that seamlessly integrates witheffective-density CMP models. Furthermore, in one embodiment of thepresent invention, the CMP process is modeled using a 2-D filterfunction.

Process of Computing Dummy Feature Density

FIG. 6 presents a flowchart that illustrates the process of computingdummy feature density in accordance with an embodiment of the presentinvention.

The process starts by discretizing the layout into a set of panels (step602). For example, the chip layout 502 can be discretized into a set ofpanels 504. In one embodiment of the present invention, the panel sizeis chosen to be small enough to avoid large discretization errors.Specifically, in one embodiment of the present invention, the panel sizeis chosen to be less than 1/10 of the filter characteristic length ofthe function that models the CMP process.

Next, the system computes feature density and slack density (step 604).Specifically, the system computes the feature density of existingfeatures 512 and the slack density that specifies the maximum density ofdummy features that can be added to the slack region 514 in the panel506. In one embodiment of the present invention, the feature density andslack density are adjusted to satisfy foundry rules. For example, thecomputed feature density can be increased so that the feature densitymeets a minimal density rule. Similarly, the computed slack density canbe decreased so that the sum of the feature density and the slackdensity meets an upper density limit.

The system then computes the dummy feature density using an iterativeprocess. Specifically, in one embodiment of the present invention, theiterative process is guided by a variance-minimizing heuristic todecrease the effective feature density variation, which is proportionalto the final post-CMP topography variation according to the CMP low-passfilter model. Note that the effective feature density is also applicableto other CMP processes, such as, copper CMP and STI CMP. In anotherembodiment of the present invention, the iterative process involvesprioritizing the panels in the set of panels.

The iterative process starts by computing the effective feature density(step 606). Note that decreasing the non-uniformity of the effectivefeature density can reduce the post-CMP topography variation.

In one embodiment of the present invention, the system computes theeffective feature density by convolving the feature density with afunction that models the CMP process. Specifically, in one embodiment ofthe present invention, the effective feature density is given by

ρ₀(i,j)=IFFT(FFT(d(i,j))·FFT(f(i,j))),

where ρ₀(i, j) is the effective feature density, d (i, j) is theexisting feature density, f (i, j) is a function that models the CMPprocess, FFT is the fast Fourier transform, and IFFT is the inverse fastFourier transform.

Furthermore, in one embodiment of the present invention, the CMP processis modeled using a 2-D filter function. For example, the CMP process canbe modeled using the 2-D filter function

f(x, y) = c₀^(⌊c₁(x² + y²)^( ^(c)2)⌋),

where c₀, c₁, and c₂ are parameters, and x, y are the 2-D coordinates.In one embodiment of the present invention c₀=0.1, c₁=−0.1, c₂=1, and x,y are measured in millimeters.

Moreover, in one embodiment of the present invention, the filterfunction is normalized so that a perfectly smooth layout in which theexisting feature density, d(i, j), is constant everywhere results in aperfectly smooth effective feature density, ρ₀(i, j), that has the samevalue as the existing feature density d(i, j).

Next, the system selects a set of panels and computes the filing amountfor the set of panels (step 608) based on a variance-minimizingheuristic. Note that selecting low effective feature density panels tofill can improve the effective feature density profile. Thevariance-minimizing heuristic is used to guide the selection of a set ofpanels and to determine the filling amount for the set of panels.Specifically, the variance-minimizing heuristic selects panels anddetermines the filling dummy density amount based on the differencebetween the target effective feature density and the effective featuredensity, i.e., diff (i, j)=t(i, j)−ρ₀(i, j), where t(i, j) is the targeteffective feature density. In one embodiment of the present invention,the variance-minimizing heuristic first calculates diff (i, j) for theplurality of panels. Next, it selects panels with large positive diff(i, j) to fill dummy density and selects the panels with large negativediff (i, j) to remove dummy density.

In one embodiment of the present invention, the variance-minimizingheuristic computes the filing amount for the selected panels using theexpression Δ(i, j)=α(t(i, j)−ρ₀(i, j)), where Δ(i, j) is the fillingamount, α is a damping factor that can be used to control the fillingspeed, and ρ₀(i, j) is the effective feature density. Note that thedamping factor is usually assigned a small value to achievesubstantially-optimal filling quality. Furthermore, to ensure that thefilling won't exceed the lower and upper limits, the system adjusts thefilling amount Δ(i, j) as follows:

Δ(i,j)=min(Δ(i,j),S(i,j)) if Δ(i,j)≧0, and

Δ(i,j)=max(−d(i,j),Δ(i,j)) if Δ(i,j)<0,

where S(i, j) is the slack density and d(i, j) is the feature density.Note that, a negative value for Δ(i, j) means that dummy features (thatwere added in previous iterations) need to be removed. These adjustmentsare made to ensure that the filling amount is bounded by the slack onthe positive side and by the current dummy feature density on thenegative side.

It will be apparent to one skilled in the art that a variety ofexpressions can be used to compute the target effective feature densityt(i, j). In one embodiment of the present invention, the targeteffective feature density t(i, j) can be specified by a user.Furthermore, in yet another embodiment of the present invention, thetarget effective feature density can be computed using the effectivefeature density. For example, in one embodiment of the presentinvention, the target effective feature density can be computed usingthe expression t(i, j)=E[ρ₀(i, j)], where E[ρ₀(i, j)] is the average ofthe effective feature density.

The system then updates the feature density, slack density, and thedummy feature density using the filling amount (step 610). Specifically,in one embodiment of the present invention, if the filling amount has apositive value, the system adds the filling amount to the dummy featuredensity and subtracts the filling amount from the slack density.Furthermore, the system adds the updated dummy feature density to thefeature density to obtain the updated feature density that will be usedfor the next iteration.

Next, the system determines whether to terminate the iterative loop ornot (step 612).

Specifically, in one embodiment of the present invention, the systemterminates the iterative loop if the dummy feature density converges orif the filling amount becomes too small. It will be apparent to oneskilled in the art that the iterative loop can be terminated based on avariety of conditions. For example, the system can decide to terminatethe iterative loop if a maximum number of iterations have been reached.

If the system determines not to terminate the iterative loop, the systemgoes back to step 606, which involves calculating the effective featuredensity. On the other hand, the process ends if the system determines toterminate the iterative loop.

FIG. 7 presents various effective feature density curves that illustratehow an iterative process can reduce the non-uniformity of the effectivefeature density in accordance with an embodiment of the presentinvention. For ease of exposition, FIG. 7 illustrates 1-D plots of theeffective feature density. Note that, these plots are for illustrationpurposes only and do not represent an actual dummy feature densitycomputation.

Curve 702 shows the “no fill” effective feature density, i.e., theeffective feature density before the iterative process starts. Note thatcurve 702 has a large amount of non-uniformity. After multipleiterations, an intermediate iterative solution is illustrated by curve704. Finally, once the iterative solution terminates, the finaliterative solution is shown by curve 706. Note that the non-uniformityof curve 704 is less than curve 702, and the non-uniformity of curve 706is less than curve 704.

Process of Adjusting the Filter Characteristic Length

FIG. 8 presents a flowchart that illustrates the process of computingdummy feature density by adjusting the filter characteristic length inaccordance with an embodiment of the present invention.

The process starts by discretizing the layout into a set of panels (step802). For example, the chip layout 502 can be discretized into a set ofpanels 504. In one embodiment of the present invention, the panel sizeis chosen to be small enough to avoid large discretization errors.Specifically, in one embodiment of the present invention, the panel sizeis chosen to be smaller than 1/10 of the filter characteristic length ofthe filter function that models the CMP process.

Next, the system computes feature density and slack density (step 804).Specifically, the system computes the feature density of existingfeatures 512 and the slack density which specifies the maximum densityof dummy features that can be added to the slack region 514 in panel506. In one embodiment of the present invention, the feature density andslack density are adjusted to satisfy foundry rules. For example, thefeature density can be increased so that the feature density meets aminimal density rule. Similarly, the slack density can be decreased sothat the sum of the feature density and the slack density meets an upperdensity limit.

The system then adjusts the filter characteristic length (step 806).

The CMP modeling function usually models the planarization effect ofCMP. As a result, the filter function is typically good at “averaging”but not good at “selecting” low-density areas for filling. In otherwords, the CMP modeling function acts as a low-pass filter that removeshigh frequency components. Note that removing high frequency componentsin the feature density can cause some details to be deleted from theplot of the effective feature density. These details may be needed bythe variance-minimizing heuristic to efficiently compute the fillingamount to reduce the non-uniformity in the effective feature density.Usually, this problem occurs more frequently when the CMP modelingfunction is a filter with a large characteristic length, because thosefilters remove more high frequency components.

In one embodiment of the present invention, the filter characteristiclength is gradually increased from a smaller value during an iterativeprocess that computes dummy feature density, until it reaches the givencharacteristic length value. Note that a filter with a smallcharacteristic length can efficiently locate and compute filling amountsfor local low-density areas. Hence, the quality of the resultingsolution for the dummy feature density assignment problem can beimproved by adjusting the filter characteristic length.

FIG. 9 presents various plots that illustrate the effect of adjustingthe filter characteristic length in accordance with an embodiment of thepresent invention. Note that the plots shown in FIG. 9 are forillustration purposes only and do not reflect actual dummy featuredensity computations that involve adjusting the characteristic length ofthe filter.

Plot 902 shows a filter function with a large characteristic length thattakes the average value of seven neighboring points. Plot 904 shows thefeature density and plot 906 shows the effective feature density, whichis obtained by convolving the feature density shown in plot 904 with thefilter function shown in plot 902. Note that the effective featuredensity plot 906 has the maximal value at position zero even though thefeature density shown in plot 904 has the minimal value at this point.Consequently, if the system computes the filling amount using a filterwith a large characteristic length, the system may erroneously fill inhigh-density areas with zero slack density to fill instead oflow-density areas.

Plot 908 shows a filter with a short characteristic length and plot 910shows the resulting effective feature density that can be computed byconvolving the feature density shown in plot 904 with the shortcharacteristic length filter function shown in plot 908. Note that,unlike the effective feature density plot 906, plot 910 has a smallvalue at position zero. This example demonstrates that by using a filterwith a small characteristic length, the system can efficiently identifyand fill low-density areas.

The system then updates the dummy feature density (step 818). It will beapparent to one skilled in the art that a variety of techniques can beused to update the dummy feature density.

In particular, in one embodiment of the present invention, the systemupdates the dummy feature density using an iterative process. Theprocess starts by computing the effective feature density (step 808).Note that post-CMP variation can be reduced by decreasing thenon-uniformity of the effective feature density. Next, the systemcalculates the filing amount for the set of panels (step 810). Thesystem then updates the feature density, slack density, and the dummyfeature density using the filling amount (step 812). Next, the systemdetermines whether to terminate the dummy filling loop or not (step814). If the system determines not to terminate the dummy-filling loop,the system goes back to step 808. On the other hand, if the systemdetermines to terminate the dummy-filling loop, the process forcomputing the dummy filling density ends for the current value of thefilter characteristic length.

Next, the system determines whether to terminate the loop thatiteratively adjusts the filter characteristic length (step 816).

If the system determines not to terminate the loop, the system goes backto step 806, which begins the next iteration by adjusting the filtercharacteristic length. On the other hand, the iterative process ends ifthe system determines to terminate the iterative loop.

CONCLUSION

The foregoing descriptions of embodiments of the present invention havebeen presented only for purposes of illustration and description. Theyare not intended to be exhaustive or to limit the present invention tothe forms disclosed. Accordingly, many modifications and variations willbe apparent to practitioners skilled in the art. Additionally, the abovedisclosure is not intended to limit the present invention. The scope ofthe present invention is defined by the appended claims.

For ease of exposition, the present application describes embodiments ofthe present invention in the context of an oxide CMP process. It will beapparent to one skilled in the art that the present invention can bereadily applied to other types of CMP processes, such as, copper CMP andShallow Trench Isolation (STI) CMP.

Furthermore, the data structures and code described in this detaileddescription are typically stored on a computer-readable storage medium,which may be any type of device or medium that can store code and/ordata for use by a computer system. This includes, but is not limited to,magnetic and optical storage devices such as disk drives, magnetic tape,CDs (compact discs) and DVDs (digital versatile discs or digital videodiscs), and computer instruction signals embodied in a transmissionmedium (with or without a carrier wave upon which the signals aremodulated). For example, the transmission medium may include acommunications network, such as the Internet.

1. A method for computing dummy feature density for a CMP(Chemical-Mechanical Polishing) process, wherein the dummy featuredensity is used to add dummy features to a layout to reduce the post-CMPtopography variation, the method comprising: discretizing a layout of anintegrated circuit into a plurality of panels; computing a featuredensity and a slack density for a panel; and computing a dummy featuredensity for the panel by, iteratively, adjusting the characteristiclength of a filter function to obtain an adjusted filter function,wherein the filter function models the CMP process; and updating thedummy feature density for the panel using the feature density, slackdensity, and the adjusted filter function; wherein the computed dummyfeature density is used to add dummy features to the layout, therebyreducing the post-CMP topography variation; wherein adjusting thecharacteristic length of the filter function improves the effectivenessof the dummy feature density computation in reducing the post-CMPtopography variation.
 2. The method of claim 1, wherein updating thedummy feature density involves: calculating an effective feature densityρ₀(i, j) for a plurality of panels using the feature density and afunction ƒ(i, j) that models the CMP process, wherein decreasing theeffective feature density variation decreases the post-CMP topographyvariation; calculating a filling amount Δ(i, j) for a set of panels inthe plurality of panels using a target effective feature density t(i,j), the effective feature density, and the slack density, and whereinthe set of panels is selected using a variance-minimizing heuristic; andupdating the feature density, the slack density, and the dummy featuredensity for the set of panels using the filling amount; wherein theconvergence rate of the dummy feature density computation is increasedbecause, in each iteration, the method updates the dummy feature densityfor the set of panels, instead of updating the dummy feature density foronly a single panel.
 3. A computer-readable storage medium storinginstructions that when executed by a computer cause the computer toperform a method for computing dummy feature density for a CMP(Chemical-Mechanical Polishing) process, wherein the dummy featuredensity is used to add dummy features to a layout to reduce the post-CMPtopography variation, the method comprising: discretizing a layout of anintegrated circuit into a plurality of panels; computing a featuredensity and a slack density for a panel; and computing a dummy featuredensity for the panel by, iteratively, adjusting the characteristiclength of a filter function to obtain an adjusted filter function,wherein the filter function models the CMP process; and updating thedummy feature density for the panel using the feature density, slackdensity, and the adjusted filter function; wherein the computed dummyfeature density is used to add dummy features to the layout, therebyreducing the post-CMP topography variation; wherein adjusting thecharacteristic length of the filter function improves the effectivenessof the dummy feature density computation in reducing the post-CMPtopography variation.
 4. The computer-readable storage medium of claim3, wherein updating the dummy feature density involves: calculating aneffective feature density ρ₀(i, j) for a plurality of panels using thefeature density and a function ƒ(i, j) that models the CMP process,wherein decreasing the effective feature density variation decreases thepost-CMP topography variation; calculating a filling amount Δ(i, j) fora set of panels in the plurality of panels using a target effectivefeature density t(i, j), the effective feature density, and the slackdensity, and wherein the set of panels is selected using avariance-minimizing heuristic; and updating the feature density, theslack density, and the dummy feature density for the set of panels usingthe filling amount; wherein the convergence rate of the dummy featuredensity computation is increased because, in each iteration, the methodupdates the dummy feature density for the set of panels, instead ofupdating the dummy feature density for only a single panel.
 5. Anapparatus for computing dummy feature density for a CMP(Chemical-Mechanical Polishing) process, the apparatus comprising: adiscretizing mechanism configured to discretize a layout of anintegrated circuit into a plurality of panels; a density-computingmechanism configured to compute a feature density and a slack densityfor a panel; and a dummy-density-computing mechanism configured tocompute a dummy feature density for the panel by, iteratively, adjustingthe characteristic length of a filter function to obtain an adjustedfilter function, wherein the filter function models the CMP process; andupdating the dummy feature density for the panel using the featuredensity, slack density, and the adjusted filter function; wherein thecomputed dummy feature density is used to add dummy features to thelayout, thereby reducing the post-CMP topography variation; whereinadjusting the characteristic length of the filter function improves theeffectiveness of the dummy feature density computation in reducing thepost-CMP topography variation.
 6. The apparatus of claim 5, wherein thedummy-density-computing mechanism is configured to: calculate aneffective feature density ρ₀(i, j) for a plurality of panels using thefeature density and a function ƒ(i, j) that models the CMP process,wherein decreasing the effective feature density variation decreases thepost-CMP topography variation; calculate a filling amount Δ(i, j) for aset of panels in the plurality of panels using a target effectivefeature density t(i, j), the effective feature density, and the slackdensity, and wherein the set of panels is selected using avariance-minimizing heuristic; and update the feature density, the slackdensity, and the dummy feature density for the set of panels using thefilling amount; wherein the convergence rate of the dummy featuredensity computation is increased because, in each iteration, the methodupdates the dummy feature density for the set of panels, instead ofupdating the dummy feature density for only a single panel.